The present invention relates to a bond for tacking an electronic part on a circuit board during soldering of the electronic part on the circuit board, and also relates to a method of soldering an electronic part with the use of such a bond.
During recent years, the technology of soldering a surface-mounting type electronic part onto a board, on which a circuit pattern is formed, has been widely used. Further, a method of soldering surface-mounting type electronic parts on opposite surfaces of a circuit board, that is, the so-called double-sided mounting method, has been prosperously used. In the case of the double-sided mounting method, soldering parts are at first formed on electrodes on the first surface (upper surface) of the circuit board, then an electronic part is mounted, its being landed on the soldering parts, and thereafter, the circuit board is heated in its entirety up to a temperature which is higher than the melting point of the soldering parts so as to melt the soldering parts in order to solder the electronic part to the first surface of the circuit board. Next, the circuit board is turned over so that the second surface thereof is faced up, and then the soldering is performed on the second surface with the use of the same process as mentioned above. In this case, there is a possibility of such a problem that the soldering part on the first surface is again melted during the second heating of the circuit board, resulting in that the electronic part on the first surface of the circuit board drops by its dead weight or vibration. In order to eliminate the above-mentioned problem, the electronic part on the first part is usually tacked on the first surface of the circuit boar by a tacking bond.
Explanation will be hereinbelow made of a conventional method of a soldering electronic part on the first surface of a circuit board with the use of a tacking bond, with reference to FIGS. 10 and 11.
Cream solder 103 is printed on the upper surfaces of electrodes 102, 102 on the circuit board 101 by using a screen printing machine. This cream solder is a mixture of solder particles having a granular size of about several ten microns, and flux, and it reduces its apparent bulk when it is melted. Next, a tacking bond 104 is applied by means of a bond applying device on the upper surface of the circuit board 101 on which the body 106 of an electronic part 105 is to be located, and then the electronic part 105 is mounted on the circuit board 101 by means of a chip mounter. With this arrangement, the body 106 of the electronic part 105 is landed on the tacking bond 104 while leads 107 horizontally extending from the body 106 are landed on the cream solder 103. Thereafter, the circuit board 101 is introduced into a heating furnace for heating the circuit board 101 so as to melt the cream solder 103. The melting point of the cream solder which is generally available is about 183 deg. C. Accordingly, the circuit board is heated up to a temperature (about 220 deg. C.) above the melting point.
By the way, the conventional tacking bond 104 is in general a mixture of synthetic resin such as epoxy resin and a hardener such as 2-methyl imidazole. The melting point of 2-methyl imidazole is about 140 deg. C. which is the hardener temperature of the conventional tacking bond 104 and which is relatively lower than the melting point (about 183 deg. C. as mentioned above) of the cream solder 103. Accordingly, if the circuit board is heated up in the heating furnace, the cream solder 103 is melted after the tacking bond 104 is hardened. Thus, the tacking bond which becomes early harder inhibits the leads of the electronic part from sinking into the melted cream solder due to its dead weight or the surface tension of the melted solder. As a result, as shown in FIG. 11, a problem occurs such that the leads 107 are not surely connected to the electrodes 102 by the solder 103. Such a problem has occurred often particularly in such a case that an electronic part 5 having very thin leads 7 laid at fine pitches is soldered.